Unusual diamagnetic response in p-wave superconductors $\rm Sr_2RuO_4$

The magnetization $\bf M$ of the $p$-wave superconductor $\rm Sr_2RuO_4$ has not been measured previously in the geometry with magnetic field $\bf H\| c$ because of the small upper critical field ($H_{c2}\sim$ 660 Oe) and low $T_c$ (1.4 K). We have used high-resolution torque magnetometry to study in detail the magnetization curves in this geometry. We find that, in the superconducting state, the $M$-$H$ curves display highly unusual hysteretic behavior. In the critical state, whenever $\rm H$ crosses zero, we observe a break in the slope $\partial M/\partial H$. In a broad field interval $[-H_0, H_0]$ bracketing zero field, $\rm M$ is reversible (to our resolution) under reversal of sweep direction. This anomalous behavior is not encountered in conventional type-II superconductors, where the critical-state behavior is always non-reversible. A possible interpretation of these unusual features is the existence of reversible edge currents. We also discuss the magnetization curves with $\bf H \| ab$, where $\rm M$ jumps sharply at $\rm H = H_{c2}$. Research supported by NSF grant DMR 0213706.